1. Field of Invention
The present invention is directed to a process for producing discontinuous silicon carbide fibers. In particular, the invention is directed to a process for producing dense discontinuous silicon carbide fibers that retain the morphology of the carbon source, are essentially devoid of boron, respond to microwave energy, and are essentially devoid of whiskers.
2. Description of the Related Art
Silicon carbide is used as reinforcement for both ceramics and plastics subjected to high temperatures. Silicon carbide materials have many desirable qualities including high resistance to oxidation, excellent mechanical strength, and the ability to withstand multiple exposures to high temperatures without deformation. The importance of such qualities has led to the development of many methods by which various shapes of silicon carbide materials are made. The different shapes are useful in a plethora of industrially important products.
Silicon carbide is commonly available in particulate, whisker, fiber, and cloth forms. Each form has distinct properties and characteristics exploitable in divers industrial applications.
Various methods have been developed to produce silicon carbide having these forms. For example, Evans, GB 998,089, describes a method for making silicon carbide cloth. First, carbon cloth is heated in an inert atmosphere, then embedded in fine powdered silicon (99.9 percent purity). The silicon-embedded cloth is then heated in an inert atmosphere to 1410° C., i.e., just below the melting point of silicon, to produce a cloth of silicon carbide.
Methods for making silicon carbide whiskers, i.e., elongated single crystals of silicon carbide, are well-known. Liquid- and gas-phase reaction systems are often used to form these elongated single crystals. Typical methods of making silicon carbide whiskers include: (1) solidification from liquid silicon carbide at high temperature and high pressure, (2) dissolving carbon into molten silicon and crystallizing the silicon carbide, (3) sublimation of silicon carbide powder and subsequent re-deposition, and (4) deposition of silicon carbide crystals from the vapor of silicon compounds.
For example, Wainer, U.S. Pat. No. 3,269,802 is directed to preparation of metal carbide products by exposing a carbonized product to an atmosphere comprising volatilizable metal-containing material, such as a metal halide or a metal carbonyl. The product takes the general form of the carbonized material, but also appears in other forms, including whiskers, fibers, and coatings. Thus, the method does not form a single product and produces environmentally undesirable waste gas.
Another method for producing metal carbide shapes is set forth in Hamling, U.S. Pat. No. 3,403,008. Organic material in the desired shape is impregnated with a metal compound solution. The impregnated form then is heated in two steps: first, to carbonize the organic material, then to form the metal carbide.
Cutler, U.S. Pat. No. 3,754,076, is directed to a method for producing silicon carbide whiskers from rice hulls, which comprise about 15–20 percent silica and carbon. A metal-containing composition, typically metal oxide, is used to catalyze the reaction. Iron and iron oxide are suitable catalysts.
Yamada, U.S. Pat. No. 4,849,196, is directed to a process for producing silicon carbide whiskers. In Yamada's method, Fe, Co, or Ni are added in any combination to minimize the production of silicon carbide powder while maximizing the yield of silicon carbide whiskers.
Weaver, U.S. Pat. No. 4,873,069, discloses a process for production of silicon carbide whiskers. In accordance with Weaver's process, discontinuous fluffy carbonized fibers (having a void volume of at least 40 percent) and ultra fine silica are heated to 1600–1900° C. for about 2 hours to produce silicon carbide whiskers. Boron oxide, alone or mixed with aluminum, serves as a catalyst. A preferred carbon source is carbonized cotton fiber having a diameter of 4–15 μm and an average length of about 2 mm. The whiskers have a smooth surface, a diameter of 0.5 to 10 μm and a length of up to 1 mm. Nixdorf, U.S. Pat. No. 5,087,272, uses the process described in Weaver to generate silicon carbide whiskers having a diameter of 1–3 microns which are then incorporated into ceramic filters for removing volatile organic compounds from gas streams.
Other methods for producing silicon carbide whiskers include use of iron to catalyze the formation of whiskers from rice hulls (Home, U.S. Pat. No. 4,283,375). Similarly, Home, U.S. Pat. No. 4,284,612, is directed to use of iron to catalyze production of silicon carbide whiskers from the combination of ground carbonized organic fibers, silica, and rice hulls.
Silicon carbide whiskers are not satisfactory for all purposes. Whiskers are very small. Therefore, whiskers are not satisfactory for applications in which relatively long fibers are preferred. For example, whiskers often are so small as to be difficult to incorporate into a fibrous web.
Whiskers also present an environmental problem. Airborne whiskers could present a health hazard. For example, the production of respirable particles from silicon carbide whisker handling, from devices containing whiskers, and in particular from filtering devices that are repeatedly exposed to high temperatures, are sources of concern. As can be seen from the methods described herein, whiskers are relatively expensive and technically difficult to make. Proper handling of whiskers is especially important so as to minimize the number of inhalable fine particles. As can be seen, therefore, when whiskers are not the desired product, it is important to avoid production of whiskers as a by-product.
Silicon carbide fiber and filament forms avoid some of the failings of silicon carbide whiskers. Woven and composited forms of silicon carbide materials may also avoid some of the problems presented by whiskers. Fiber, filament, and woven forms comprise particles larger than whiskers, and are therefore, less likely to yield airborne respirable particles. However, the prior art does not include a suitable method to produce such products essentially without whiskers.
Wei, U.S. Pat. No. 4,481,179, is directed to a method of producing silicon carbide bonded fiber composites, starting from a carbon-bonded carbon fiber composite. Galasso, U.S. Pat. No. 3,640,693, is directed to forming a silicon-containing fiber by casting silicon metal in a glass tube, drawing composite filaments, removing the glass sheath, then exposing the silicon metal to carbon or nitrogen to produce silicon carbide or silicon nitride, respectively. Debolt, U.S. Pat. No. 4,127,659, is directed to coating a refractory substance, such as carbon, with silicon carbide by chemical vapor deposition to produce a silicon carbide filament containing a core and a coating of carbon-rich silicon carbide. Srinivasan, U.S. Pat. No. 5,729,033, is directed to a method of producing silicon carbide material (fiber, fabric, or yarn) by carbothermal reduction of silicon material. Particular proportions of silica and carbon are preferred.
DeLeeuw, U.S. Pat. No. 5,071,600 and U.S. Pat. No. 5,268,336, are directed to methods for producing silicon carbide fibers by the reaction of polycarbosilane and methylpolydisilylazane resins in the presence of boron. Tokutomi, U.S. Pat. No. 5,344,709, describes a silicon carbide fiber produced from polycarbosilane fiber and having an amorphous layer of carbon thereon. Yajima, U.S. Pat. No. 4,100,233, describes a method of producing continuous silicon carbide fibers which involves dissolving or melting an organosilicon compound in a solvent and spinning the solution into filaments. The spun filaments are then heated to volatilize low molecular weight compounds, and, finally, baked to form silicon carbide fibers.
SYNTHESIS OF SIC-BASED FIBERS DERIVED FROM HYBRID POLYMER OF POLYCARBOSILANE AND POLYVINYLSILANE, Proceedings of the International Symposium on Novel Synthesis and Processing of Ceramics, 107–112 (1997), A. Idesaki, M. Narisawa, K. Okamura, M. Sugimoto, T. Seguchi, and M. Itoh, discloses a fiber comprising oxygen, silicon, and carbon. Polycarbosilane or a mixed polycarbosilane/polymethylsilane solution was partially crosslinked by heating, then melt-spun. The melt-spun material then is cured by heating in air at least to 1,000° C. to carbonize the material. The resultant product is identified as silicon carbide fiber. However, this is a misnomer because the product comprises between about 6 and 13 wt percent oxygen. The presence of oxygen in the backbone of this so-called silicon carbide fiber renders it unsuitable, especially in a moist environment.
Each of these methods has disadvantages. The continuous silicon carbide filaments produced by the chemical vapor deposition method are not homogenous and, when chopped to obtain fibers, a carbon core is exposed. The resultant fiber product has reduced resistance to oxidation. All of the polymer conversion methods are disadvantageous in that they require synthesis of the starting material which must then be spun, cured, and pyrrolized to burn off the organic material. The submicron silicon carbide powder process incorporated by reference in Srinivasan is expensive and difficult to implement because the polymer carrier requires further processing to effectuate its removal.
Okada, U.S. Pat. No. 5,618,510, discloses a method for producing silicon carbide fiber, sheets, and three-dimensional articles having a silicon nitride coating. Carbon fiber is activated in a known manner. The porous activated carbon material having a specific surface area of 100 to 2500 m2/g is treated with silicon monoxide gas at a temperature of 800 to 2000° C. Pressure during silicidation must be 10 Pa or less to fully convert the carbon and prevent formation of whiskers. The resulting silicon carbide fiber material then is heat treated in nitrogen in the absence of oxygen to reduce porosity of the surface and to coat the material with silicon nitrides. The product has an oxygen content of 1.0 wt percent and a nitrogen content of 2.0 wt percent.
PREPARATION OF SILICON CARBIDE FIBER FROM ACTIVATED CARBON FIBER AND GASEOUS SILICON MONOXIDE, Okada, K., H. Kato, R. Kubo, and K. Nakajima Ceramic Engineering and Science Proceedings 16(4): 45–54, 1995, discloses manufacture of silicon carbide fiber from activated carbon fiber having specific surface area of 500 to 2500 m2/g, and exemplified use of carbon fiber having a diameter of 10 microns and a specific surface area of 960 m2/g. The carbon fiber is reacted with silicon monoxide at 10 Pa pressure. The resultant silicon carbide fiber was treated in oxygen at 800° C. to combust materials other than silicon carbide. The resultant silicon carbide fiber shows a granular structure not present in the activated carbon fiber. The specific surface area was reduced from 960 to 50 m2/g due to growth in pore size. Although photomicrographs indicate that granularity was reduced, the density appears to have remained the same, as the dimensions of the fibers appear to remain unchanged. The particles are described as more dense after treatment with nitrogen at 1600° C. Although silicon nitride was not found by x-ray diffraction, the nitrogen content of the particles was 2.0 wt percent. The nitrogen content is found in the 3 micron surface layer, thus essentially permeating the 10 micron fiber, or approximately 85 percent by weight.
Okada, U.S. Pat. No. 5,676,918, discloses a method for producing silicon carbide fiber having a high tensile strength and a uniform structure without whiskers. Activated, porous carbon fiber having a specific surface area of 100 to 3000 m2/g and length at least 5 mm are reacted with silicon monoxide at 800–2000° C. at a pressure no more than 100 Pa. The fibers must be held in tension to produce the desired result. The low pressure is required to reduce whisker formation. The silicon carbide fiber has the same form as the carbon fiber, and the dimensions of the silicon carbide fiber and the carbon fiber are essentially unchanged. Silicon carbide fiber produced from unactivated, non-porous carbon fiber having specific surface areas lower than 100 m2/g remained unreacted at the core.
Nakajima, U.S. Pat. No. 5,922,300, discloses a method for converting porous carbon filaments, yarns, and woven and non-woven fabrics having a specific surface area of 300 to 2000 m2/g to silicon carbide fiber. The carbon filaments are mixed with a silicon-containing powder, such as silicon or silicon dioxide, and heated to 1200 to 1500° C. The patent discloses that whisker-producing catalysts, such as iron, must not be present, and the pressure is 1000 Pa or less to prevent whisker formation. The resultant silicon carbide fiber product is separated from the remaining particles by sieving or washing. The resultant silicon carbide fiber product has essentially the same dimensions (length and diameter) as the carbon fiber, and thus has a relatively low density. Example 1 discloses that Renoves® A-10 is suitable as activated carbon fiber precursor. This material, commercially available from Osaka Gas K.K., has a specific surface area of 1100 m2/g and a pore volume of 0.54 cm3/g. Therefore, the calculated apparent density, as that term is defined herein, of the exemplified product of Example 1 is 1.17 g/cc.
Okada, U.S. Pat. No. 6,316,051, discloses a method for manufacturing silicon carbide fiber, yarn, or fabric by reaction of activated carbon fiber having a specific surface area of 700 to 1500 m2/g with silicon or silicon monoxide powder at 1200–1500° C. under reduced pressure. The fibers then are treated with a boron-containing substance in an amount sufficient to provide at least 0.1 wt percent boron and heat-treated at a temperature of 1700–2300° C. Whisker production is minimized by removing volatiles from the activated carbon fiber. Silicon carbide fiber product has the same structure as the carbon fiber from which it was made, as does the product heat-treated without boron. However, the boron-containing, heat-treated product is said to have a higher density.
Nixdorf, U.S. Pat. No. 6,767,523, discloses a method for producing silicon carbide fiber having up to 1 wt percent whiskers. Carbonized cotton fibers and fumed silica powder are reacted in the presence of ferrous sulfate and calcium oxalate at about 1700° C. According to the patent, carbonized cotton fiber is the sole carbon source that will produce silicon carbide fiber having up to 1 wt percent whiskers, and no other carbonized organic fiber will work in this method. The resultant fibers retain the morphology of the carbon fiber, and so remain not very dense. The size of the silicon carbide fiber is limited by the length of the chopped carbon fiber derived from cotton. The length is short, about ⅛ to ½ inch.
Silicon carbide fiber products produced by these methods are not completely satisfactory. The resultant silicon carbide fiber is porous and not dense, is dense only at the surface, or is contaminated with densifying agents such as nitrogen, boron, or silicon nitride. Some products are limited in size by the limitations of the raw materials and include undesirable whiskers. Whiskers are air pollutants and must be controlled to minimize health problems, especially of those who handle them.
Thus, there remains a need for an easily implemented, economical, and environmentally benign method of producing homogenous, dense, discontinuous, silicon carbide fibers essentially devoid of whiskers.